1. Technical Field
The present invention relates to an image processing method of transforming input image data to generate output image data for display, a program for allowing a computer to execute the method, a recording medium recording the program, and a projector.
2. Related Art
In projection-type display apparatuses such as projectors, in order to cope with display deformation due to gradient projection or the like, trapezoidal distortion correction functions are provided to transform input images in advance, so that rectangular images can be displayed correctly on projection faces.
In addition, in direct-view type image display apparatuses other than the projection-type display apparatuses, input images are enlarged or reduced to be displayed,
However, there are cases where the quality of an output image is deteriorated compared to that of an input image when this kind of transformation is processed. For example, when a data component having a high spatial frequency is included in the input image data and the data component is transformed to have a higher spatial frequency, there may be a case where the transformed component becomes out of the display capacity of the display device.
In this case, a component having a high spatial frequency component is bent toward the low frequency side at the nyquist frequency of the display device to be viewed by an observer as a moire pattern, thereby deteriorating the image quality of an output image.
Although the reason for the occurrence of the moire is relatively simple as described above, It is very difficult to detect an occurrence of the moire in advance in, a practical application and to suppress the deterioration of the image quality to a minimal level.
In other words, in order to detect the occurrence of the moire in advance, the calculation of a performed transformation operation, input image data, and all the spatial frequency response characteristics of a display device are required. However, the amount of calculation is very large, which makes it impractical to detect the occurrence of the moire.
As a method of easily detecting the occurrence of the moire, for example, in a fixed pixel type image pickup device, a method in which a focus-matching image and a defocused image are compared and when there is a marked difference between the focus-matching image which includes high frequency data and the defocused image which doest not include high frequency data, it is automatically determined that the moire occurs is proposed (for example, see JP-A-6-197260 (paragraph [0033], FIG. 7), JP-A-10-276350 (paragraph [0020], FIG. 4), and JP-A-2001-86355 (paragraphs [0035] to [0037], FIG. 6)).
In the technology disclosed in JP-A-6-197260, when the occurrence of the moire is determined, a process of subtracting a moire component from the focus-matching data is performed. To be more specifically, the defocused image is subtracted from the focus-matching position image including the moire component, and the result of the subtraction is assumed to be a moire component, and the moire component is subtracted from the focus-matching position image for reducing the moire component.
On the other hand, in the technology disclosed in JP-A-10-276350, when the occurrence of the moire is determined, a process of subtracting a low frequency component of the moire component from the focus matching data in the frequency domain is performed. To be more specifically, a process of subtracting the frequency component of the moire is performed using a low band-limiting filter and a band-limiting filter for reducing the moire component.
In addition, when a grey scale image is converted into halftone dots, in order to detect the occurrence of a moire, a process of converting into two halftone dots is performed, wherein the correspondence with data is half phase out of phase in one halftone dot for halftoning and, and the two halftone data are compared with each other (for example, see JP-A-2001-86355). When the moire occurs, the phase is moved rapidly by the above-described process, and accordingly there is a marked difference between two images. The comparison is performed on an image on which a defocus process and an averaging process of the halftone dots have been performed, and accordingly, it is possible to detect the occurrence of the moire.
However, when the technologies disclosed in the JP-A-6-197260, JP-A-10-276350, and JP-A-2001-86355 are applied to an image display apparatus on a premise that a transformation operation is performed on the projection-type display apparatus or the like, there is the following problem.
In other words, according to the technologies in the past, an image in which the moire occurs and the target image basically have a same size and a same shape, so that the correlating the position or size of the moire to the target image can be easily performed, and accordingly, the technique of subtracting the occurred moire component from the target image can be easily applied.
On the other hand, since the moire of an image display apparatus such as a projection-type display apparatus is generated as a result of transformation and it is very difficult to correlate the position or size of the generated moire to input image data. As a result, it is very difficult to perform the process of subtracting the generated moire component from the input image data.
In addition, when the method of using a low-limiting filter or a band-limiting filter which is used in the technology disclosed in JP-A-10-276350 is used, it is impossible to differentiate a frequency component and a moire component which are inherently included in the input image data, and accordingly, there is a possibility of deterioration of the image quality.